Camera onPreviewFrame not called on XT910

I have a problem with the camera of my MOTO XT910 android phone. I would like only to access to the camera's (frame)buffer but "onPreviewFrame" is never called. On my emulator, it works fine.

Thanks in advance,

My code is as below:

public class Store extends SurfaceView implements SurfaceHolder.Callback, PreviewCallback {
 static {
     System.loadLibrary("hello-jni");
 }
 
 public native void decode(Bitmap pTarget, byte[] pSource);
 
 private Camera mCamera;
 private byte[] mVideoSource;
 private Bitmap mBackBuffer;
 private Paint mPaint;
 private Size lSize;
 private static String TAG = Store.class.getCanonicalName();
 
 
 public Store(Context context) {
     super(context);
     getHolder().addCallback(this);
     setWillNotDraw(false);
 }
 
 public void surfaceCreated(SurfaceHolder holder) {
     try {
         mCamera = Camera.open();
         mCamera.setDisplayOrientation(0);
         mCamera.setPreviewDisplay(null);
         mCamera.setPreviewCallbackWithBuffer(this);
         Log.d(TAG,"surfaceCreated ok");
     } catch (IOException eIOException) {
         mCamera.release();
         mCamera = null;
         Log.d(TAG,"surfaceCreated failed");
         throw new IllegalStateException();
     }
 }
 
 public void surfaceChanged(SurfaceHolder pHolder, int pFormat, int pWidth, int pHeight) {
     Log.d(TAG,"surfaceChanged in");
     mCamera.stopPreview();
     lSize = findBestResolution(pWidth, pHeight);        
     invalidate();       
     PixelFormat lPixelFormat = new PixelFormat();
     PixelFormat.getPixelFormatInfo(mCamera.getParameters()
             .getPreviewFormat(), lPixelFormat);
     int lSourceSize = lSize.width * lSize.height * lPixelFormat.bitsPerPixel / 8;
     mVideoSource = new byte[lSourceSize];
     mBackBuffer = Bitmap.createBitmap(lSize.width, lSize.height,Bitmap.Config.ARGB_8888);
     Camera.Parameters lParameters = mCamera.getParameters();
     lParameters.setPreviewSize(lSize.width, lSize.height);
     lParameters.setPreviewFormat(PixelFormat.YCbCr_420_SP);
     mCamera.setParameters(lParameters);
     mCamera.addCallbackBuffer(mVideoSource);
     mCamera.startPreview();
     Log.d(TAG,"surfaceChanged out");
 }
 
 private Size findBestResolution(int pWidth, int pHeight) {
     List<Size> lSizes = mCamera.getParameters().getSupportedPreviewSizes();
     Size lSelectedSize = mCamera.new Size(0, 0);
     for (Size lSize : lSizes) {
         if ((lSize.width <= pWidth) && (lSize.height <= pHeight)
                 && (lSize.width >= lSelectedSize.width)
                 && (lSize.height >= lSelectedSize.height)) {
             lSelectedSize = lSize;
         }
     }
     if ((lSelectedSize.width == 0) || (lSelectedSize.height == 0)) {
         lSelectedSize = lSizes.get(0);
     }
     return lSelectedSize;
 }
 
 public void surfaceDestroyed(SurfaceHolder holder) {
     if (mCamera != null) {
         Log.d(TAG,"surfaceDestroyed");
         mCamera.stopPreview();
         mCamera.release();
 
         mCamera = null;
         mVideoSource = null;
         mBackBuffer = null;
     }
 }
 
 public void onPreviewFrame(byte[] pData, Camera pCamera) {
     Log.d(TAG,"onPreviewFrame");
     decode(mBackBuffer, pData);
     invalidate();
 }
 
 @Override
 protected void onDraw(Canvas pCanvas) {
     Log.d(TAG,"onDraw in");
     if (mCamera != null) {
         Paint paint = new Paint(); 
         paint.setColor(Color.YELLOW); 
         String text = String.format("%dx%d", lSize.width, lSize.height);
         pCanvas.drawText(text, 10, 10, paint);          
         pCanvas.drawLine(0, 0, 100, 100, paint);
         pCanvas.drawLine(20, 0, 0, 20, paint); 
         pCanvas.drawBitmap(mBackBuffer, 0, 0, mPaint);
 
         mCamera.addCallbackBuffer(mVideoSource);
         Log.d(TAG,"onDraw out");
     }
 }
 

}

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